@reactivex/rxjs/compat/operator/elementAt.ts

import { Observable } from 'rxjs';
import { elementAt as higherOrder } from 'rxjs/operators';

/**
 * Emits the single value at the specified `index` in a sequence of emissions
 * from the source Observable.
 *
 * <span class="informal">Emits only the i-th value, then completes.</span>
 *
 * <img src="./img/elementAt.png" width="100%">
 *
 * `elementAt` returns an Observable that emits the item at the specified
 * `index` in the source Observable, or a default value if that `index` is out
 * of range and the `default` argument is provided. If the `default` argument is
 * not given and the `index` is out of range, the output Observable will emit an
 * `ArgumentOutOfRangeError` error.
 *
 * @example <caption>Emit only the third click event</caption>
 * var clicks = Rx.Observable.fromEvent(document, 'click');
 * var result = clicks.elementAt(2);
 * result.subscribe(x => console.log(x));
 *
 * // Results in:
 * // click 1 = nothing
 * // click 2 = nothing
 * // click 3 = MouseEvent object logged to console
 *
 * @see {@link first}
 * @see {@link last}
 * @see {@link skip}
 * @see {@link single}
 * @see {@link take}
 *
 * @throws {ArgumentOutOfRangeError} When using `elementAt(i)`, it delivers an
 * ArgumentOutOrRangeError to the Observer's `error` callback if `i < 0` or the
 * Observable has completed before emitting the i-th `next` notification.
 *
 * @param {number} index Is the number `i` for the i-th source emission that has
 * happened since the subscription, starting from the number `0`.
 * @param {T} [defaultValue] The default value returned for missing indices.
 * @return {Observable} An Observable that emits a single item, if it is found.
 * Otherwise, will emit the default value if given. If not, then emits an error.
 * @method elementAt
 * @owner Observable
 */
export function elementAt<T>(this: Observable<T>, index: number, defaultValue?: T): Observable<T> {
  return higherOrder.apply(undefined, arguments)(this);
}